The present invention relates generally to rechargeable cells containing liquid sodium and sulfur and more particularly to a corrosion resistant cathode container and current collector structure for sodium-sulfur cells.
In the operation of a sodium-sulfur cell, both the sodium and sulfur are liquid at the operating temperature of the cell. Sodium ions from the liquid sodium anode are transported through a ceramic electrolyte to the liquid sulfur cathode and react with sulfur to form sodium polysulfides according to the reversible reaction, EQU 2Na+xS.revreaction.Na.sub.2 S.sub.x (x=3, 4, or 5).
The sodium ions surrender charge in the reaction, and the current generated thereby is conducted through the sulfur and supporting wick material to a metallic container which is connected to a terminal. The metallic container is typically stainless steel with a chromium coating on the inner surface thereof to impede corrosion of the container by sodium polysulfides formed during cell operation. The chromium coating eventually corrodes by reaction with polysulfides to form NaCrS.sub.2 which seriously impairs performance of the cell, particularly through deterioration of the electrolyte.
The invention solves or substantially reduces in critical importance the foregoing problem with existing sodium-sulfur cell structures by providing a reliable corrosion resistant container and current collector structure for the liquid sulfur cathode. The invention comprises a metallic outer container for the cathode having on the inner surface thereof a coating of electrically insulating, nonreactive glass. Adjacent the glass coating is a tubular metallic cathode current collector.
It is therefore a principal object of the invention to provide an improved liquid sodium-sulfur cell structure.
It is another object of the invention to provide a corrosion resistant cathode container structure for sodium-sulfur cells.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.